Investigation of Copper and Zinc
Contamination on the Work Piece Surface
with WEDM
V. K. Manupati1, G. Rajyalakshmi1, M. L. R. Varela2(&),
J. Machado3, and G. D. Putnik2
1
Department of Manufacturing, School of Mechanical Engineering,
VIT, Vellore, India
manupativijay@gmail.com, rajyalakshmi@vit.ac.in
2
Department of Production and Systems, University of Minho,
Guimarães, Portugal
{leonilde,putnikgd}@dps.uminho.pt
3
Department of Mechanical Engineering, School of Engineering,
University of Minho, Guimarães, Portugal
jmachado@dem.uminho.pt
Abstract. This work presents an investigation on Wire Electric Discharge
Machining (WEDM) of Pure Titanium (Grade-2). Pure Titanium and its alloys
are the most suitable metallic materials for biomedical applications. In health
care, they are used for implant devices like knee and hip implants, fixing
materials for bones like nails and screws, and in the oral and maxillofacial
surgery. Properties like good biocompatibility, high corrosion resistance, and
high fracture strength make titanium parts the perfect solution for bio-implants.
The more frequent use of the titanium necessitates an economical machining
process. Conventional processes like milling, grinding or drilling have their
limits because of the material’s mechanical properties which cause high tool
wear. WEDM proves to be an alternative especially for manufacturing complex
parts as there is no actual contact with tool and the work piece. Within this
study, the capabilities of Wire-EDM for the manufacturing of clean surfaces
with distinct Surface Roughness (SR) and Machining Speed (MS) are conducted
through optimization of process parameters. Taguchi technique is used for
optimization and the experiments are conducted according to Taguchi Design of
Experiments (DOE). Different wire electrode materials have been used considering a possible copper and zinc contamination on the work piece surface.
Keywords: WEDM
Titanium grade 2 Taguchi Design Optimization
1 Introduction
Increased use of titanium and its alloys as biomaterials stems from their lower modulus,
superior biocompatibility, and better corrosion resistance when compared to more
conventional stainless and cobalt-based alloys. As a hard tissue replacement, the low
elastic modulus of titanium and its alloys is generally viewed as a biomechanical
© Springer International Publishing AG, part of Springer Nature 2019
J. Machado et al. (Eds.): HELIX 2018, LNEE 505, pp. 608–615, 2019.
https://doi.org/10.1007/978-3-319-91334-6_83
Investigation of Copper and Zinc Contamination
609
advantage because the less elastic modulus can result in lower stress shielding. Hence,
for the present research pure Titanium has been chosen as the test material.
Titanium and its alloys are often used in the medical sector. It is used for joint
substitutions like knee and hip implants, fixing materials for bones like nails and screws,
and in the oral and maxillofacial surgery because of the materials’ characteristics like
good biocompatibility and bio adhesion in addition to corrosion resistance and specific
mechanical properties. However, problems arise during manufacturing of titanium
implants with conventional processes like milling, grinding or drilling obtain their limits
because of the materials mechanical properties which cause e.g. high tool wear.
Electro Discharge Machining (EDM) represents an alternative especially for
manufacturing of filigree parts. Furthermore, it is applicable for flexible manufacturing
process that allows to machine implants which can be adapted to specific patients’
demands. In terms of a single-unit production, it is hereby an economic process.
Further differences between EDM and conventional manufacturing processes can be
pointed out in terms of surface integrity. Through machining by cutting processes
smearing and plastic deformation (e.g. micro ploughing) might lead to small cavities.
These negative surface conditions do not occur after machining by EDM because of a
nearly force free process. This leads to a surface which can be very well sterilised.
Especially the Wire-EDM process leaves residues like copper and zinc on surfaces.
These residues substantially affect the biocompatibility. One way to decrease or
eliminate this problem is to use non-common wire electrodes. These are electrodes
made of materials which themselves feature a high biocompatibility. Another option
could be to use Trilayered wires but these are very expensive to produce or even
non-producible. Figure 1 shows the schematic diagram of the Wire Electric Discharge
Machining (WEDM) process.
Fig. 1. Wire EDM process diagram Tsai et al., (2003)
Advantages of WEDM Process can be summarized through: as continuously
travelling wire is used as the negative electrode, so electrode fabrication is not required
as in EDM; there is no direct contact between the work piece and the wire, eliminating
610
V. K. Manupati et al.
the mechanical stresses during machining; WEDM process can be applied to all
electrically conducting metals and alloys irrespective of their melting points, hardness,
toughness or brittleness; and users can run their work pieces over night or over the
weekend unattended.
Disadvantages of WEDM Process, can be summarizes through: high capital cost
that is required for WEDM process; there is a problem regarding the formation of recast
layer; WEDM process exhibits very slow cutting rate; and it is not applicable to very
large work piece.
This paper focuses on a basic study on the capabilities of Wire-EDM for the
manufacturing of clean surfaces with distinct roughness and a high surface integrity.
Common wire materials will be analysed regarding a possible copper and zinc contamination of the work piece surface due to re-solidification of small particles originating from the wire. Furthermore, the process itself will be optimised by parametric
optimization of various process parameters to obtain distinct surface roughness and
better machining speed of WEDM.
2 Literature Review
Electric Discharge Machining amongst the many non-conventional machining methods
has numerous applications in the current industrial processes. The EDM process
converts the electrical energy generated in a channel between a cathode and an anode to
thermal energy which creates sufficient high temperature for the erosion of work piece
material [1]. This distinct feature of thermo-electric conversion of EDM makes it
suitable for manufacturing of complex parts particularly for aerospace and automotive
industry, and surgical instruments [2]. In EDM process, the desired shape of the object
is obtained by electrical discharges i.e., sparks. A localised spark is produced between
the work-piece and the electrode in the presence of a dielectric fluid. Here, generally
deionized water acts as a dielectric fluid which filters and directs the sparks. The water
also acts as a coolant and also helps in flushing away the residual metal particles [3]. In
Wire EDM, a moving metallic wire is used as the electrode to cut a programmed
contour. The wire movement is controlled numerically which provides accuracy for
machining of complex two and three dimensional work piece configurations.
Wire EDM is a non-contact type machining method which provides an economical and
efficient way for cutting Metal Matrix Composite materials. The work material Hardness does not possess any disadvantage in cutting as there is no relative contact
between the wire and the work material [4]. Instead of cutting the material, the WEDM
produces high temperature which melts or vaporizes the material, leaving very little
debris providing accurate results. A taut thin wire discharges the electrified current
which acts as a cathode guided alongside the desired cutting path. The work piece
undergoing machining is usually fixed on a table. High accuracy instrumentation
comprising of CNC systems is used to control the movements of work piece which
reduces the positioning errors to a large extent [5].
Some of the common wire electrode is made up of copper because of its higher
electrical conductivity but it wears rapidly and its tension ability is poor [6].
Investigation of Copper and Zinc Contamination
611
From literature, it has been found that many researchers have focused on the
developments of EDM, WEDM, and micro-WEDM. Hence, in this study, an attempt
has been made to determine the effect of process parameters and wire material on the
responses like MRR and surface roughness. Taguchi’s orthogonal array has been used
for conducting the experiments and presented the results of wire materials influence on
performance of WEDM process for Titanium grade 2.
3 Design of Experiments
Design of experiments (DOE) or experimental design is the design of any information
gathering exercises where variation is present, whether under the full control of the
experimenter or not. Classical experimental design often requires a large number of
experiments to be carried out when number of machining parameters increase.
Genichi Taguchi developed this statistical method for improving the quality of the
products and manufacturing goods which finds many applications in the field of
engineering [7].
This method provides an efficient, systematic and simplistic approach for optimization of the design for better performance, higher quality and cost cutting. This
method is more effective when used with the discrete and qualitative design parameters.
The optimization process in a Taguchi method is carried out in 3 steps: System
design, parameter design, and tolerance design. The main thrust of this method lies in
the usage of a method for product and process design i.e. Parameter design, which
focuses on deciding the parameters for developing the best of quality characteristics with
minimal variations. The S/N ratio is used to measure the quality characteristics deviating
from the desired values. The analysis includes 3 categories of quality characteristics:
(i) Lower-the-better, (ii) Higher-the-better, and (iii) Nominal-the-better. The greatest
S/N ratio value should be selected for best quality characteristics. In this study, 4
machining parameters were used as control factors and each parameter was designed to
Table 1. Taguchi’s L9 orthogonal
array.
Exp. no. T
ON
1
1
2
1
3
1
4
2
5
2
6
2
7
3
8
3
9
3
T
OFF
1
2
3
1
2
3
1
2
3
IP Wire
feed
1 1
2 2
3 3
2 3
3 1
1 2
3 2
1 3
2 1
Table 2. Process parameters for WEDM and their
considered levels.
Symbol Control
factor
Unit
Level
1
Level
2
Level
3
A
µs
50
70
90
µs
4
6
9
A
3
4
5
mm/min 50
70
90
B
C
D
Pulse on
time (T
ON)
Pulse off
time (T
OFF)
Current
(IP)
Wire
feed
612
V. K. Manupati et al.
Table 3. Properties of different types of wire used.
Material Coating Tensile
Elongation Conductivity Diameter
22% IACS 0.25 mm
Coated wire
Cu Zn 36 Zn
900 N/mm2 1.50%
Annealed wire Cu Zn 37 Zn
900 N/mm2 2%
22% IACS 0.25 mm
Table 4. Experimental readings of SR, MS and S/N ratio values for machining speed and SR.
Wire
Exp. no. T
ON
T
IP Wire
OFF
feed
MS
(mm/min)
SR (lm) S/N ratio MS
Ra
S/N ratio SR
Coated
wire
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1.24
1.18
0.27
0.5
0.498
0.59
0.85
0.78
0.28
1.112
1.37
0.608
0.392
0.18
0.57
0.82
0.435
0.16
11
8.511
9.2376
10.0069
8.895
10.659
7.5138
8.3414
7.3834
11.5695
9.2105
11.3269
8.5086
9.176
8.6392
8.2458
7.8961
8.4904
−20.8278537
−8.59961181
−9.31118306
−20.0059912
−8.98291905
−0.55432924
−7.51719262
−8.42477895
−7.36512794
−1.26629181
−9.28566414
−1.08222133
−8.59716215
−19.2530681
−8.72947056
−8.32465594
−17.9482528
−8.57856302
Annealed
wire
1
1
1
2
2
2
3
3
3
1
1
1
2
2
2
3
3
3
1
2
3
2
3
1
3
1
2
1
2
3
2
3
1
3
1
2
1
2
3
3
1
2
2
3
1
1
2
3
3
1
2
2
3
1
1.868433703
1.437640146
−11.37272472
−6.020599913
−6.055413145
−4.582959767
−1.411621486
−2.158107946
−11.05683937
0.922095745
2.734411343
−4.321928415
−8.13427866
−14.8945499
−4.882502887
−1.723722952
−7.230214861
−15.91760035
have 3 levels. According to the Taguchi quality design concept, a L9 orthogonal arrays
table with 9 rows (corresponding to the number of experiments) was chosen for the
experiments. Table 1 shows the distribution of levels of different parameters in a
Taguchi Orthogonal Array. In Tables 2 and 3 the properties of the wires and process
parameters of the WEDM is shown. The analysis using Taguchi is portrayed in Table 4.
4 Method of Analysis
A single response optimization is carried out to investigate the effects of machining
parameters on Machining Speed (MS) and Surface Roughness (SR). According to the
Taguchi method, S/N ratios in Eqs. 1 and 2 were calculated for each experiment. The
objective of optimization is to maximize the MS and minimize the SR. The response
table for S/N ratios of MS is calculated considering the fact that MS is a larger-the-better
performance characteristic; the maximization of the quality characteristic of interest is
sought and is expressed as:
Investigation of Copper and Zinc Contamination
X
1 n 1
S=NRatio ¼ 10 log
n i¼1 y2
613
ð1Þ
yij = observed response value i = 1, 2… …n; j = 1, 2 … k, n = number of replications.
The surface roughness is the lesser-the-better performance characteristic and the
S/N ratio for SR is calculated by:
S=NRatio ¼ 10 log
X
1 n 2
y
n i¼1 ij
ð2Þ
SEM and EDX analysis are performed on the samples obtained from single
objective optimization of Surface Roughness of both Coated and Annealed wire. The
objective of the analysis is to check the contamination levels of copper and other
materials in the sample after wire electric discharge machining. Figure 2, shows the
SEM results and composition of different materials that are present on the surface of the
machined surface which are cut through coated wire for three spectrums shown in
Table 5.
Fig. 2. (a) SEM micrograph for spectrum 1, 2 and 3 (b) composition graph for spectrum 1, 2 and
3, (c) EDX analysis for spectrum 1, 2 and 3, for coated wire, respectively.
According to SEM images of sample of coated wire, the fibrous network is having
maximum Cu content as 11.0%, and in the globular phases, the Cu content is reduced
to 9.58%.
Figure 2 shows the SEM results and composition of different materials that are
present on the surface of the machined surface which are cut through annealed wire for
three spectrums shown in Table 5 respectively. According to SEM images of sample of
annealed wire, the surface appears as beach sand shaped with globular phases over it.
There are some cracks visible to across the whole surface. At various points, EDAX
analysis were performed. In the globular phases, more amount of Cu (4.72%) is seen.
In the remaining matrix, the Cu content is reduced, which tells that there has been
614
V. K. Manupati et al.
Table 5. Chemical composition of sample for all spectrums.
micro-segregation of Cu, with Cu accumulating in the globular phases. In the matrix
dense fibrous network of the alloy is seen Fig. 2. Moreover, we can realise that the Cu
content is reduced (3.11%), along with some Cu less phases.
5 Conclusions
The present work focuses on optimization of parameters of wire electric discharge
machining and their effects of different wires to improve the surface quality (reduce
contamination), surface characteristics (distinct surface roughness), and time of operation (higher machining speed). The material used in this study is pure Titanium (Grade
2) owing to its better biocompatibility. Various parameters affecting the machining
conditions are considered for single and multi-response optimization. The conclusions
obtained are as follows. From single response optimization, it is observed that
machining through annealed wire provides higher machining speed. However, it also
results in higher surface roughness. The SEM and EDAX results show that the contamination levels in annealed wire, especially of copper, is significantly lower than the
Zinc coated wire. This is due to better diffusion of zinc and copper in the annealed wire
which avoids spark erosion from the wire to the material. Overall, it is recommended to
use annealed wire during Wire Electric Discharge Machining for biological applications as it provides better results in terms of both machining speed and
biocompatibility.
Future work can be performed by studying effects of other types of wire like
molybdenum wire, brass wire and composite wires for obtaining better results. Different biocompatible materials like titanium alloys, stainless steel, magnesium alloys
can also can be studied in terms of better machinability and surface characteristics.
Acknowledgements. This work is supported by Fundação para a Ciência e Tecnologia with ref.
PEst2015-2020.
Investigation of Copper and Zinc Contamination
615
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